This invention relates to the repair of boiling water reactor components which have been damaged or weakened by stress corrosion cracking. In particular, the invention relates to the repair of cracked beans in the top guide of a boiling water reactor.
A conventional boiling water reactor (BWR) comprises a reactor pressure vessel (RPV) filled with cooling water, a core of nuclear fuel submerged in that water and a stainless steel cylinder, called the core shroud, which surrounds the nuclear fuel core and is concentric with the RPV walls The core shroud comprises a shroud flange for supporting the shroud heads, a circular cylindrical upper shroud wall having a top guide welded to the shroud flange, an annular top guide support ring welded to the bottom rim of the upper shroud wall, a circular cylindrical middle shroud wall comprising multiple sections welded in a stack, with a top rim of the middle shroud wall being welded to the top guide support ring, and an annular core plate support ring welded to the bottom rim of the middle shroud wall and to the top rim of a lower shroud wall. The entire shroud is supported by a shroud support, which is welded to the bottom rim of the lower shroud wall, and by an annular shroud support plate, which is welded at its inner diameter to the shroud support and at its outer diameter to the RPV wall. Reactor water flows down the annular space between the RPV wall and the shroud, around the lower rim of the shroud and up through the fuel core located within and surrounded by the cylindrical shroud.
The fuel core consists of a multiplicity of upright and parallel fuel bundle assemblies arranged in 2xc3x972 arrays, each assembly consisting of an array of fuel rods inside a Zircaloy fuel channel. The assemblies of each array are separated by a cruciform gap which allows vertical travel of a cruciform control rod blade in between the fuel channels. Each control rod blade contains neutron-absorbing material. The power level is maintained or adjusted by positioning control rods up and down within the core while the fuel bundle assemblies are held stationary. Each array of fuel bundle assemblies is supported at the top by a top guide and at the bottom by a core plate. In particular, the top guide provides lateral support to the upper end of the fuel assemblies, neutron monitoring instrument assemblies and installed neutron sources, and maintains the correct fuel channel spacing to permit control rod insertion. The top guide is designed so that during periodic refueling operations, the fuel bundle assemblies can be lifted out of and lowered into the core without removing the top guide.
One type of top guide installed in certain types of BWRs has a fabricated design comprising a lattice of interlocking upper and lower beams held together by a large circular ring. The circular ring of the top guide sits on the top guide support ring of the shroud, and is provided with radially inwardly directed flanges that capture the distal ends of the beams. The beams and support ring are typically made of Type 304 stainless steel with high carbon content. The composition of standard Type 304 stainless steel is 18.0-20.0 wt. % Cr, 8.0-10.5 wt. % Ni, 2.0 wt. % Mn, 1.0 wt. % Si, 0.08 wt. % C, 0.045 wt. % P and 0.03 wt. % S.
The foregoing top guide design contains many creviced welded and unwelded connections, which are used to attach the lattice beam supports to the inner surface of the support ring and for rigid span support of the top guide structure over the core. The Type 304 stainless steel with high carbon content which is typically used in early BWR plants, in conjunction with the many creviced regions, results in the top guide being susceptible to intergranular stress corrosion cracking (IGSCC) and irradiation-assisted stress corrosion cracking (IASCC). Sustained exposure to conditions conducive to IGSCC and IASCC will eventually require repairs or complete top guide replacement.
The present invention is a device employed to repair damaged top guides having cracks in the beam lattice region. The invention permits use of a limited number of local repairs to the top guide without welding. Therefore, employment of the invention avoids the need for complete top guide replacement. Thus, this invention is expected to benefit the end user by facilitating in-reactor repair with reduced costs and reduced downtime.
The repair in accordance with the invention entails the installation of a cruciform lattice segment which reinforces the damaged or weakened region of the beam lattice. This cruciform lattice segment is held in place atop the existing top guide with specially designed straps. The cruciform lattice segment and associated straps are arranged so that a beam segment of the cruciform lattice segment bridges the weakened region in the cracked top guide. Thus, the bridging beam segment transmits loads across the weakened region of the top guide. The straps are designed to avoid interference with removal and installation of the fuel assemblies.